<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Title</title>
</head>
<body>
<input type="text" id="i1">
<input type="text" id="i2">
<input type="button" value="加" onclick="f(1)">
<input type="button" value="减" onclick="f(2)">
<input type="button" value="乘" onclick="f(3)">
<input type="button" value="除" onclick="f(4)">
<h3>
  结果:<span></span>
</h3>
<script>
  //得到两个文本框
  let i1 = document.querySelector("#i1");
  let i2 = document.querySelector("#i2");
  let s = document.querySelector("span");
  function f(x) {
    if (isNaN(i1.value)||isNaN(i2.value)){
      s.innerText = "输入错误!";
      return;
    }
    switch (x) {
        case 1:
          s.innerText = i1.value*1+i2.value*1;
          break;
      case 2:
        s.innerText = i1.value-i2.value;
        break;
      case 3:
        s.innerText = i1.value*i2.value;
        break;
      case 4:
        s.innerText = i1.value/i2.value;
        break;
    }

  }
</script>
</body>
</html>